1. Field of the Invention
The present invention relates to organic electroluminescent devices, and more particularly, to an organic electroluminescent device with an improved sealing structure and a method of manufacturing the organic electroluminescent device.
2. Description of the Related Art
Recently, electroluminescent (EL) devices regarded as self-luminous type display devices are receiving a lot of attention as a next-generation display device due to advantages of a wide viewing angle and good contrast and rapid response characteristics. EL devices are classified into inorganic EL devices and organic EL devices depending on the material of the emissive layer. Organic EL devices can realize color display and have better luminance and response characteristics than inorganic EL devices.
In the manufacture of an organic electroluminescent device, anodes are formed on a glass substrate or another type of transparent insulating substrate in a predetermined pattern, and an organic layer and cathodes are sequentially deposited on the anodes such that the cathodes intersect the anodes. The organic layer includes a hole transporting layer as the lowermost layer, an emissive layer, and an electron transporting layer, which are sequentially stacked using organic compounds. Suitable organic compounds for these organic layers include copper phthalocyanine (CuPc), N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benxidine (NPB), tris-8-hydroxyquinoline aluminum (Alq3) and the like.
As a voltage is applied across the anode and cathode of an organic EL device having the structure described above, holes injected from the anode are transported across the hole transporting layer to the emissive layer. Electrons injected from the cathode are transported across the electron transporting layer to the emissive layer. Excitons are generated in the emissive layer by recombination of electrons and holes. As the excitons transit from an excited state to a ground state, fluorescent molecules of the emissive layer emit light, and thus an image can be displayed.
Organic materials used in such an organic EL device described above are adversely affected by moisture and oxygen in the air. Moisture and oxygen degrade the characteristics of the organic material, cause delaminating of the cathode, and reduce the lifetime of the organic EL device. For these reasons, the organic EL device is encapsulated to protect the organic layer from moisture and impurities present in the air, as illustrated in FIGS. 1 through 4.
In particular, as illustrated in FIGS. 1 and 2, the rear surface of a transparent substrate 22 on which an organic layer 21 has been formed is sealed with a metal cap 23 or a glass cap 24 using a sealant 25. In an alternative method, multiple layers of sealant are coated on the rear surface of the transparent substrate 22 to form an encapsulation layer 26 for the organic layer 21 formed on the rear surface, as illustrated in FIG. 3. In another alternative method, as illustrated in FIG. 4, at least one sealing layer 27 is formed to cover the organic layer 21 formed on the rear surface of the transparent substrate 22, and covered once again with a cap 28 attached to the transparent substrate 22.
As examples of the sealing structures described above, U.S. Pat. No. 5,059,862, U.S. Pat. No. 5,047,687, and U.S. Pat. No. 5,059,861 disclose sealing structures with a capping layer on the organic layer. Japanese Laid-open Patent Publication No. hei 9-274990 discloses a sealing structure in which the organic layer is sealed with a polyurethane sealing layer and the sealing layer is encapsulated with an external sealing layer containing a moisture absorbent. U.S. Pat. No. 5,882,761 discloses a sealing structure in which a glass sealing case is mounted on the rear surface of a transparent substrate to seal the organic layer formed thereon, a moisture absorbent is placed inside the glass sealing case, and the space formed between the glass sealing case and the transparent substrate is filled with inert gas.
In the above-described sealing structures for organic EL devices, the sealant used to attach a metal cap or glass cap to the transparent substrate may contaminate the organic layer. In particular, to seal the organic layer with the metal cap or glass cap, the sealant is initially applied on a bonding site and pressed with the metal cap or glass cap to cover the organic layer. During this sealing process, the sealant is squeezed and spreads out toward the inside and the outside of the sealed space of the organic EL device. When the sealant spread out into the sealing space reaches the active area of the organic layer, the organic layer is deteriorated by the solvent of the sealant. When electrode pads to apply a voltage to the organic layer are interposed at the bonding site of the transparent substrate with the metal cap or glass cap, the solvent of the sealant easily permeates the organic layer along the electrode pads and contaminates or corrodes the external protruding ends of the electrode pads. This contamination or corrosion of the external protruding electrode ends increases adhesion of the electrode pads to the edge of the ground substrate and thus relatively decreases adhesion to a flexible printed circuit board, which should be connected to the electrode pads to apply a voltage.
In the manufacture of organic EL devices, a number of organic EL devices are simultaneously manufactured in a single large substrate and then separated into individual devices by cutting. Therefore, the spread of the sealant over the cutting sites may cause failures in the cutting process. This undesired spread of the sealant significantly degrades the quality of the organic EL device in terms of lifetime, color purity, and luminance.
To address this problem, Korean Laid-open Patent Publication No. 2000-10172 discloses an example of an organic EL device. This suggested organic EL device has a barrier inside the sealant layer to prevent the sealant from spreading over an emissive portion 30, as illustrated in FIG. 5. In particular, the organic EL device includes a lower substrate 31 having the emissive portion 30 on its top surface, an upper substrate 32 disposed above the lower substrate 31 with a predetermined gap, an internal barrier 33 formed along the edges of the upper substrate 32 and the lower substrate 31 to seal the space therebetween, and a sealant layer 34 attached to the outside wall of the internal barrier 33 to combine the upper substrate 32 and the lower substrate 31.
In the organic EL device illustrated in FIG. 5, the internal barrier 33 should be formed to a height no less than 0.5 mm so that the internal space is high enough to protect the emission layer 30 from external impacts and large enough to accommodate a moisture absorbent therein. It is difficult to form such a tall internal barrier using a photoresist method or silk-screen printing technique. To form such a tall internal barrier described above, repeated depositions are required, which degrades the shape or intensity of the resulting internal barrier.
When a flexible printed circuit board is connected to the lower substrate 31, spreading of the sealant out of the sealing structure cannot be effectively suppressed, thereby causing failures in a subsequent substrate cutting process. In addition, the sealing structure described above does not ensure an external margin large enough to bind the flexible printed circuit board and the electrode pads together. In connecting the flexible printed circuit board to the electrode pads of the organic EL device, the electrode pad area is not fully covered by the flexible printed circuit board so that the ends of the electrode pads are exposed in the air. Therefore, there is a problem in that the exposed ends of the electrode pads act as passages along which moisture or foreign materials enter the organic layer.